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Abstract:

A spring wire with hardness of 50 to 56 HRC is subjected to first and
second shot peening processes within a warm working temperature range of
150 to 350° C. In the first shot peening process, the first shot
of a shot size of 1.0 mm or more is used. In the second shot peening
process, the second shot smaller in shot size than the first shot is
used. Through these shot peening processes, compressive residual stress
is imparted to the spring wire. The spring wire includes a residual
stress increase part, residual stress peak part, and residual stress
decrease part. In the residual stress decrease part, a part including the
compressive residual stress magnitude of which is equivalent to the
magnitude of the compressive residual stress at the surface of the spring
wire exists at a position at a depth exceeding the permissible pit depth.

Claims:

1. A coil spring for vehicle suspension made of a spring wire to which
compressive residual stress is imparted by shot peening characterized by
comprising:a residual stress increase part in which the compressive
residual stress increases from the surface of the spring wire in a depth
direction;a residual stress peak part in which the compressive residual
stress becomes the maximum; anda residual stress decrease part in which
the compressive residual stress decreases from the residual stress peak
part in the depth direction of the spring wire, whereinin the residual
stress decrease part, a part comprising the compressive residual stress
magnitude of which is equivalent to the magnitude of the compressive
residual stress at the surface of the spring wire exists at a position at
a depth exceeding the permissible pit depth of the spring wire.

2. The coil spring for vehicle suspension according to claim 1, whereinthe
permissible pit depth is 0.25 mm, and compressive residual stress of -400
MPa or more is imparted to a region from the surface of the spring wire
to position at the permissible pit depth.

3. The coil spring for vehicle suspension according to claim 2, whereinthe
hardness of the spring wire is 50 to 56 HRC.

4. A method for manufacturing a coil spring for vehicle suspension
characterized by comprising:a bending process of forming a spring wire
constituted of a spring steel into a helical shape;a first shot peening
process configured to impart compressive residual stress to the spring
wire by applying the spring wire with the first shot of a shot size of
1.0 mm or more; anda second shot peening process configured to applies
the spring wire with the second shot smaller in shot size than the first
shot after the first shot peening process, whereinby the first shot
peening process, and the second shot peening process, a residual stress
increase part in which the compressive residual stress increases from the
surface of the spring wire in a depth direction, a residual stress peak
part in which the compressive residual stress becomes the maximum, and a
residual stress decrease part in which the compressive residual stress
decreases from the residual stress peak part in the depth direction of
the spring wire are generated, andin the residual stress decrease part, a
part comprising the compressive residual stress magnitude of which is
equivalent to the magnitude of the compressive residual stress at the
surface of the spring wire is generated at a position at a depth
exceeding the permissible pit depth of the spring wire.

5. The manufacturing method according to claim 4, whereinthe first shot
peening process and the second shot peening process are carried out in a
state where the spring wire is kept at a processing temperature of 150 to
350.degree. C.

6. The manufacturing method according to claim 5, whereinthe processing
temperature in the first shot peening process is higher than the
processing temperature in the second shot peening process.

7. The manufacturing method according to claim 6, whereinthe kinetic
energy of the first shot in the first shot peening process is greater
than the kinetic energy of the second shot in the second shot peening
process.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001]This application is based upon and claims the benefit of priority
from prior Japanese Patent. Application No. 2009-144460, filed Jun. 17,
2009, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002]1. Field of the Invention

[0003]The present invention relates to a coil spring for vehicle
suspension used for a suspension mechanism of a vehicle such as an
automobile or the like, excellent in corrosion durability, and method for
manufacturing the same.

[0004]2. Description of the Related Art

[0005]It is known that a coil spring for suspension mechanism is subject
to an influence of an antifreeze agent scattered on a road in the
wintertime or the like. The antifreeze agent contains salt, and hence the
agent promotes corrosion of the surface of the coil spring constituted of
spring steel. It is known that a corrosion pit (etch pit) particularly
exerts a great influence upon the durability of the coil spring. For
example, as shown in FIG. 11, part of the surface 1a of the spring wire 1
is corroded to form a hole-like shape by moisture or salt, whereby a
corrosion pit 2 is formed. Although the shape of the corrosion pit 2
varies, there is even a corrosion pit 2 having a cross section as
schematically shown in, for example, FIG. 12.

[0006]As shown in FIG. 13, when the corrosion pit 2 reaches a depth R of a
certain level or more, and the weight of the vehicle is continuously
applied to the coil spring, a fatigue crack 3 occurs at the bottom part
2a or the like of the pit 2. When the crack 3 grows large, the coil
spring is broken. In other words, even when, the coil spring is used in a
corrosive environment, the coil spring can be used without breakage if
the size of the corrosion pit is within the permissible pit depth
Rmax.

[0007]On the other hand, it is desired that the coil spring for suspension
be used at higher stress in order to effect weight reduction of a
vehicle. In order to realize the higher stress in the coil spring, it
becomes important to impart compressive residual stress to the vicinity
of the surface of the spring wire. It is known from the past that by
subjecting a coil spring to shot peening, compressive residual stress is
imparted to the vicinity of the surface of the coil spring, and the
durability thereof is enhanced. For example, in Jpn. Pat. Appln. KOKAI
Publication No. 2000-345238 or Jpn. Pat. Appln. KOKAI Publication No.
2008-106365, multistage shot peening is disclosed. In the multistage shot
peening, shot peening is carried cut a plurality of times in a dividing
mariner.

[0008]As means for producing compressive residual stress in a region from
the surface of the spring to a deep position, stress peening and warm
peening (hot peening) are known. In the stress peening, the shot is
applied to the coil spring in a state where the spring is compressed. In
the warm peening, the shot is applied to the coil spring in a state where
the spring is heated at a temperature of about 250° C. In
comparison with the ordinary shot peening to be carried out at room
temperature, in the stress peening or warm peening, it is possible to
cause compressive residual stress to appear in a region up to a deep
position in the material. However, the stress peening requires equipments
configured to compress the coil spring. Furthermore, in the stress
peening, the shot is applied to the coil spring in a state where the coil
spring is compressed, and hence gaps between spring wire parts become
small. Accordingly, there is a problem that it is hard for the shot to
hit the inside of the coil spring or positions between the spring wire
parts.

[0009]In a conventional coil spring for suspension, large compressive
residual stress is imparted to the vicinity of the surface of the spring
by shot peening. However, when the coil spring for suspension is used in
a corrosive environment in which a corrosion pit occurs, the coil spring
breaks in some cases after a relatively short period of use by the growth
of the corrosion pit. Thus, it is also proposed to improve the corrosion
resistance, and suppress occurrence of the corrosion pit and growth of
the corrosion pit by contriving the components of the material (spring
steel).

[0010]For example, the corrosion resistance of the coil spring is improved
by adding an alloy element such as Ni, Cr, Mo or the like to the spring
steel. However, the spring steel containing such an alloy element is
expensive, this being a cause making the cost of the coil spring high.
Further, once the size of the corrosion pit reaches the permissible pit
depth, there is the possibility of the coil spring being broken while
making a fatigue crack occurring at a bottom part or the like of the
corrosion pit a starting point.

BRIEF SUMMARY OF THE INVENTION

[0011]An object of the present invention is to provide a coil spring for
vehicle suspension capable of enhancing the corrosion durability, and
being used at higher stress, and method for manufacturing the same.

[0012]A coil spring for vehicle suspension of the present invention is
that made of a spring wire to which compressive residual stress is
imparted by shot peening and comprises a residual stress increase part,
residual stress peak part, and residual stress decrease part. In the
residual stress increase part, the compressive residual stress increases
from the surface of the spring wire in a depth direction. In the residual
stress peak part, the compressive residual stress becomes the maximum. In
the residual stress decrease part, the compressive residual stress
decreases from the residual stress peak part in the depth direction of
the spring wire. Furthermore, in the coil spring, in the residual stress
decrease part, a part comprising the compressive residual stress
magnitude of which is equivalent to the magnitude of the compressive
residual stress at the surface of the spring wire exists at a position at
a depth exceeding the permissible pit depth of the spring wire.

[0013]According to the coil spring for vehicle suspension of the present
invention, even when the corrosion pit grows to a position near the
permissible pit depth, it is possible to prevent a fatigue crack from
occurring at the bottom part or the like of the corrosion pit, and
enhance the corrosion durability. As a result of this, it becomes
possible to use the suspension coil spring for vehicle suspension at
higher stress, and effect weight reduction of the vehicle.

[0014]When the permissible pit depth of the coil spring is 0.25 mm, it is
desirable that compressive residual stress of -400 MPa or more (implying
an absolute value of 400 MPa or more, the same is true of the following)
be imparted to a region from the surface of the spring wire to a position
at the permissible pit depth. As the hardness of the spring wire, the
hardness of 50 to 56 HRC is recommendable. Further, it is desirable that
a position which is located deeper than the residual stress peak part,
and at which the compressive residual stress begins to largely lower be
located deeper than 0.2 mm from the surface of the spring. Further, it is
also desirable that compressive residual stress of -400 MPa or more be
imparted to a region from the surface to a position at a depth of 0.3 mm.

[0015]A method for manufacturing a coil spring for vehicle suspension of
the present invention comprises a bending process, first shot peening
process, and second shot peening process. In the bending process, a
spring wire constituted of a spring steel is formed into a helical shape.
In the first shot peening process, compressive residual stress is
imparted to the spring wire by applying the spring wire with the first
shot of a shot size of 1.0 mm or more. The second shot peening process is
carried out after the first shot peening process. In the second shot
peening process, the spring wire is applied with the second shot smaller
in shot size than the first shot. By the first shot peening process, and
second shot peening process, a residual stress increase part, residual
stress peak part, and residual stress decrease part produced in the
spring wire, and in the residual stress decrease part, a part comprising
the compressive residual stress magnitude of which is equivalent to the
magnitude of the compressive residual stress at the surface of the spring
wire is generated at a position at a depth exceeding the permissible pit
depth of the spring wire.

[0016]According to the manufacturing method of the present invention, it
is possible to cause a high level of compressive residual stress
effective in preventing occurrence of a fatigue crack and development of
the crack to appear in a region from the surface of the spring wire to a
position at a depth exceeding the permissible pit depth. Furthermore, it
is possible to make a difference between the compressive residual stress
near the surface of the spring wire and compressive residual stress near
the bottom part of the corrosion pit small. As a result of this, it is
possible to obtain compressive residual stress distribution highly
effective in preventing a fatigue crack from occurring in the corrosion
pit.

[0017]In the present invention, it is recommendable to carry out the first
shot peening process and second shot peening process in a state where the
spring wire is kept at a processing temperature of 150 to 350° C.
The processing temperature in the first shot peening process is higher
than the processing temperature in the second shot peening process.
Further, it is advisable to make the kinetic energy of the first shot
greater than the kinetic energy of the second shot.

[0018]Additional objects and advantages of the invention will be set forth
in the description which follows, and in part will be obvious from the
description, or may be learned by practice of the invention. The objects
and advantages of the invention may be realized and obtained by means of
the instrumentalities and combinations particularly pointed out
hereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0019]The accompanying drawings, which are incorporated in and constitute
a part of the specification, illustrate embodiments of the invention, and
together with the general description given above and the detailed
description of the embodiments given below, serve to explain the
principles of the invention.

[0020]FIG. 1 is a side view of part of an automobile provided with a coil
spring for suspension according to an embodiment of the present
invention;

[0021]FIG. 2 is a perspective view of a coil spring for suspension shown
in FIG. 1;

[0022]FIG. 3 is a flowchart showing an example of manufacturing processes
of the coil spring for suspension shown in FIG. 2;

[0023]FIG. 4 is a flowchart showing another example of manufacturing
processes of the coil spring for suspension shown in FIG. 2;

[0024]FIG. 5 is a graph showing the compressive residual stress
distribution of example 1 according to the present invention;

[0025]FIG. 6 is a graph showing the compressive residual stress
distribution of example 2 according to the present invention;

[0026]FIG. 7 is a graph showing the compressive residual stress
distribution of example 3 according to the present invention;

[0028]FIG. 9 is a graph showing the corrosion life of each of examples 1
and 2 according to the present invention, and comparative example 1;

[0029]FIG. 10 is a graph showing the corrosion life of each of examples 4
and 5 according to the present invention, and comparative example 2;

[0030]FIG. 11 is a cross-sectional view schematically showing an example
of a corrosion pit;

[0031]FIG. 12 is a cross-sectional view schematically showing another
example of a corrosion pit; and

[0032]FIG. 13 is a cross-sectional view schematically showing a corrosion
pit in which a crack has occurred.

DETAILED DESCRIPTION OF THE INVENTION

[0033]A coil spring for vehicle suspension according to an embodiment of
the present invention, and method for manufacturing the coil spring will
be described below with reference to the drawings.

[0034]A suspension mechanism 11 of a vehicle 10 shown in FIG. 1 is
provided with a coil spring 12 for vehicle suspension (hereinafter
referred to simply as a coil spring 12), and shock absorber 13. In the
coil spring 12 shown in FIG. 2, a spring wire 20 is formed into a helical
shape. The coil spring 12 elastically supports the load of the vehicle 10
in a state where the spring 12 is compressed in the direction of the axis
X.

[0035]An example of the coil spring 12 is a cylindrical coil spring. An
example of a wire diameter d (shown in FIG. 2) of the spring wire 20 is
12.5 mm. An average coil diameter D is 110.0 mm, free length (length at
no load) is 382 mm, number of active turns is 5.39, and spring constant
is 33.3 N/mm. Although the main stream of the wire diameter of the coil
spring 12 is 8 to 21 mm, diameters other than the above may also be
employed. Further, coil springs of various forms such as a barrel-shaped
spring, double headed conical spring, tapered coil spring, variable pitch
coil spring, load axis control coil spring, and the like may also be
employed.

[0037]FIG. 3 shows manufacturing processes of a hot-formed coil spring. In
a heating process S1, a spring wire which is a material for the coil
spring is heated at an austenitizing temperature (higher than A3
transformation point, and lower than 1150° C.). The heated spring
wire is bent into a helical shape in a bending process (coiling process)
S2. Thereafter, the coiled spring wire is subjected to heat treatment
such as a quenching process S3, tempering process S4, and the like.

[0038]The spring wire is thermal refined by the heat treatment so that the
hardness of 50 to 56 HRC can be obtained. For example, a coil spring of
design maximum stress of 1300 MPa is thermal refined so that hardness of
54.5 HRC can be obtained. A coil spring of design thermal refined so that
hardness of 53.5 HRC can be obtained. Further, in a hot setting process
S5, a load in the axial direction of the coil spring is applied to the
coil spring for a predetermined time. The hot setting process S5 is
carried out as warm working by utilizing the remaining heat after the
heat treatment.

[0039]Thereafter, a first shot peening process S6 is carried out. In the
first shot peening process S6, the first shot (cut wire pieces made of
iron) having a shot size (grain size) of 1.0 mm is used. The spring wire
is applied with the first shot at a processing temperature of 230°
C., at a velocity of 76.7 m/s, and with kinetic energy of
12.11×10-3 J. As a result of this, compressive residual stress
appears in a wide region from the surface of the spring wire in the depth
direction. The compressive residual stress distribution will be described
later in detail. It is desirable that the surface roughness of the spring
wire obtained by the first shot peening process S6 be 75 μm or less.
It should be noted that the velocity of the shot is a value obtained by
multiplying the circumferential velocity obtained from the diameter of
the impeller of the shot peening machine, and rotational speed of the
impeller by 1.3. For example, when the diameter of the impeller is 490
mm, and rotational speed of the impeller is 2300 rpm, the velocity of the
shot becomes 1.3×0.49×3.14×2300/60=76.7 m/s.

[0040]After the first shot peening process S6 is carried out, a second
shot peening process S7 is carried out. In the second shot peening
process S7, the second shot smaller in size than the first shot is used.
The shot size of the second shot is 0.67 mm. The spring wire is applied
with the second shot at a processing temperature of 200° C., at a
velocity of 46 m/s, and with kinetic energy of 1.31×10-3 J.

[0041]In the second shot peening process S7, the second shot smaller in
size than that of the first shot in the first shot peening 56 is used.
Furthermore, the velocity of the second shot in the second shot peening
process S7 is smaller than the velocity of the first shot in the first
shot peening process S6. As a result of this, the large surface roughness
of the spring wire after the first shot peening process S6 can be made
small by the second shot peening process S7, and the surface state of the
spring wire is improved. It should be noted that as another example of
the second shot peening process S7, the spring wire may be applied with
the second shot having a shot size of 0.40 mm at a processing temperature
of 200° C., at a velocity of 86.7 m/s, and with kinetic energy of
0.99×10-3 J.

[0042]Table 1 shows data in which kinetic energy values of the shots are
compared with each other in terms of shot peening conditions. When the
shot size is larger, the kinetic energy becomes larger even if the same
velocity is employed. For example, when the large-grain shot having a
shot size of 1 mm is used, the kinetic energy becomes about 1.5 times
that of the shot having a shot size of 0.87 mm. When the large-grain shot
having a shot size of 1.1 mm is used, the kinetic energy becomes about
twice that of the shot having the shot size of 0.87 mm. Conversely, when
the small-grain shot having a shot size of 0.67 mm is used, the kinetic
energy becomes less than half that of the shot having the shot size of
0.87 mm. When the shot having a shot size of 0.4 mm is used, the kinetic
energy becomes less than that of the shot having the shot size of 0.67 mm
even if the velocity is made about twice.

[0043]In each of all the cases including example 1, and examples 2 to 5 to
be described later, the kinetic energy of the first shot of the first
shot peening process S6 is made greater than the kinetic energy of the
second shot of the second shot peening process S7.

[0044]As the processing temperature in each of the first shot peening
process 56 and second shot peening process S7, a temperature within a
range of 150 to 350° C. is suitable. That is, these processes are
warm peening (hot peening) processes utilizing the remaining heat after
the heat treatment. Furthermore, the second shot peening process 67 is
carried out at a processing temperature lower than the first shot peening
process S6.

[0045]According to the shot peening processes S6 and S7 of example 1, it
is possible to produce large compressive residual stress from the surface
to the deep position without compressing the coil spring unlike in the
conventional stress peening. As a result of this, equipments configured
to compress the coil spring are made unnecessary unlike in the case of
stress peening. Furthermore, intervals between the spring wire parts are
not narrowed unlike in the stress peening, and hence it is possible for
the shot to sufficiently hit the inside of the coil spring or positions
between spring wire parts.

[0046]After the two stages of the shot peening processes S6 and S7 have
been carried out, a presetting process S8 and painting process S9 are
carried out. Thereafter, in order to inspect the external appearance and
characteristics of the coil spring, an inspection process S10 is
conducted. It should be noted that the presetting process S8 may be
omitted.

[0047]FIG. 4 shows a manufacturing process of a case where the coil spring
is coiled in the cold working. As shown in FIG. 4, the spring wire before
being subjected to the coiling process is subjected in advance to heat
treatment such as a quenching process S11, tempering process 512, and the
like. The spring wire is formed into a helical shape in a bending process
(coiling process) S13 to be carried out as cold working. Thereafter, in a
stress relief annealing process S14, the coil spring is left as it is in
an atmosphere of a predetermined temperature for a predetermined time,
whereby the processing strain caused during the formation time is
eliminated.

[0048]Thereafter, in this manufacturing process shown in FIG. 4 as in the
case of the hot-formed coil spring of FIG. 3, the hot setting process S5,
first shot peening process S6, second shot peening process S7, presetting
process S8, painting process S9, and inspection process S10 are carried
out. It should be noted that the coil spring may also be coiled in the
warm working. It should be noted that the presetting process S8 may be
omitted.

[0049]FIG. 5 shows the compressive residual stress distribution of the
coil spring of example 1. The abscissa of FIG. 5 represents positions
from the surface of the spring wire in the depth direction. Although the
ordinate of FIG. 5 represents the residual stress, in accordance with the
customs of the technical field, the compressive residual stress values
are expressed as negative values.

[0050]As shown in FIG. 5, the compressive residual stress of the coil
spring of example 1 includes a residual stress increase part 11, high
stress part 12, residual stress peak part T3, and residual stress
decrease part 14. In the residual stress increase part T1, the
compressive residual stress increases from the surface of the spring wire
toward the inside of the spring wire in the depth direction. In the high
stress part T2, the compressive residual stress is maintained at a high
level. In the residual stress peak part T3, the compressive residual
stress becomes the maximum. In the residual stress decrease part T4, the
compressive residual stress decreases from the residual stress peak part
T3 in the depth direction of the spring wire. Furthermore, in the coil
spring of this example 1, in the residual stress decrease part T4, a part
B having the compressive residual stress magnitude of which is equivalent
to the magnitude of the compressive residual stress value A at the
surface of the spring wire exists at a position at a depth exceeding the
permissible pit depth Rmax of the spring wire.

[0051]Here, the permissible pit depth Rmax implies the maximum pit
depth associated with the possibility of the coil spring for suspension
having the compressive residual stress distribution obtained by the
conventional shot peening being broken while making a fatigue crack
occurring at a bottom part or the like of the corrosion pit a starting
point. In the conventional coil spring, when the corrosion pit depth gets
close to 0.2 to 0.25 mm, the coil spring is broken with a high
probability. The permissible pit depth in this case is 0.25 mm.

[0052]As shown in FIG. 5, in the coil spring of example 1, the part B
having the compressive residual stress magnitude of which is equivalent
to the magnitude of the compressive residual stress value A at the
surface of the spring wire exists at a position at a depth exceeding the
permissible pit depth Rmax of the spring wire. Furthermore, the coil
spring of example 1 has compressive residual stress of -400 MPa or more
in a wide region from the surface to a position at a depth exceeding the
permissible pit depth Rmax.

[0053]In each of all the cases including example 1, and examples 2 to 5 to
be described later, two-stage shot peening (warm double shot peening)
constituted of the first shot peening process S6 and second shot peening
process 37 is carried out. That is, by the first shot peening process S6
of the first stage, the peak of the compressive residual stress appears
at a position deep from the surface and, moreover the compressive
residual stress occurs up to the deep position. Further, by the second
shot peening process S7 of the second stage, it is possible to enhance
the compressive residual stress near the surface as indicated by the
arrow h in FIG. 5. In this way, it is possible to obtain the high stress
part T2 in which the compressive residual stress is maintained at a high
level in a region from the vicinity of the surface to a deep position.

[0054]It should be noted that when the spring wire is heated in the
atmosphere, the surface thereof is decarbonized, and the hardness of the
surface is made lower than the inside by about 125 HV. The strength of
the compressive residual stress is proportional to the spring hardness.
That is, as the spring hardness becomes smaller, the compressive residual
stress also becomes smaller. When the spring hardness is 515 HV which is
the lower limit of the hardness range of 50 to 56 HRC (515 to 615 HV),
and the minimum hardness of the surface at which decarbonization has been
caused is 390 HV, the compressive residual stress value of the surface at
that time is set at about -400 MPa or more.

Example 2

[0055]The type of steel of the spring wire is the high corrosion resistant
spring steel (spring steel S) identical with example 1. The manufacturing
processes are identical with example 1 except for the size of the shot
used in a first shot peening process S6. In this example 2, the first
shot of a shot size of 1.1 mm was used in the first shot peening process
S6. After the first shot peening process S6, a second shot peening
process S was carried out by using the second shot having a shot size of
0.67 mm. The velocity of the shot and processing temperature were
identical with example 1.

[0056]FIG. 6 shows the compressive residual stress distribution of example
2. The coil spring of example 2 also has, like that of example 1, a
residual stress increase part T1, high stress part T2, residual stress
peak part T3, and residual stress decrease part T4. As described
previously, in the residual stress increase part T1, the compressive
residual stress increases from the surface of the spring wire in the
depth direction. In the high stress part T2, the compressive residual
stress is maintained at a high level. In the residual stress peak part
T3, the compressive residual stress becomes the maximum. In the residual
stress decrease part T4, the compressive residual stress decreases from
the residual stress peak part T3 in the depth direction of the spring
wire. In the coil spring of example 2, as in the case of example 1, a
part B having the compressive residual stress magnitude of which is
equivalent to the magnitude of the compressive residual stress value A at
the surface of the spring wire exists at a position at a depth exceeding
the permissible pit depth Rmax of the spring wire. Furthermore, the
coil spring of example 2 has compressive residual stress of -400 MPa or
more in a wide region from the surface to a position at a depth exceeding
the permissible pit depth Rmax.

Example 3

[0057]The type of steel of the spring wire is the high corrosion resistant
spring steel (spring steel S) identical with example 1. The manufacturing
processes are identical with example 2 except that high-frequency heating
is used for heat treatment of the spring wire. In this example 3, the
spring wire is heated by high-frequency heating in a quenching process
S3, whereby the surface of the spring wire is prevented from being
decarbonized. In a first shot peening process S6, the first shot having a
shot size of 1.1 mm was used. In a second shot peening process S7, the
second shot having a shot size of 0.67 mm was used. The velocity of the
shot and processing temperature were identical with example 1.

[0058]FIG. 7 shows the compressive residual stress distribution of example
3. The coil spring of example 3 also has, like those of examples 1 and 2,
a residual stress increase part T1, high stress part T2, residual stress
peak part T3, and residual stress decrease part T4. Further, in the
residual stress decrease part T4, a part B having the compressive
residual stress magnitude of which is equivalent to the magnitude of the
compressive residual stress value A at the surface of the spring wire
exists at a position at a depth exceeding the permissible pit depth
Rmax of the spring wire. Furthermore, the coil spring of example 3
ha-compressive residual stress of -400 MPa or more in a wide region from
the surface to a position at a depth exceeding the permissible pit depth
Rmax.

Example 4

[0059]The steel type of SAE 9254 was used as the spring wire. The chemical
components (mass %) of SAE 9254 are C: 0.51 to 0.59, Si: 1.20 to 1.60,
Mn: 0.60 to 0.80, Cr: 0.60 to 0.80, S: 0.040 max., P: 0.030 max., and Fe:
remnant. The manufacturing processes are identical with example 1. In
this example 4, the spring wire constituted of SAE 9254 (hardness: 53.5
HRC) was subjected to a first shot peening process (velocity: 76 m/s,
processing temperature: 230° C.) by using the first shot having a
shot size of 1.0 mm. Thereafter, the spring wire was subjected to a
second shot peening process (velocity: 46 m/s, processing temperature:
200° C.) by using the second shot having a shot size of 0.67 mm.

Example 5

[0060]The steel type of SAE 9254 was used as the spring wire, and a coil
spring was manufactured by the same processes as example 2. That is, in
example 5, the spring wire constituted of SAE 9254 (hardness: 53.5 HRC)
was subjected to a first shot peening process (velocity: 76 m/s,
processing temperature: 230° C.) by using the first shot having a
shot size of 1.1 mm. Thereafter, the spring wire was subjected to a
second shot peening process (velocity: 46 m/s, processing temperature:
200° C.) by using the second shot having a shot size of 0.67 mm.

Comparative example 1

[0061]As the spring wire, the high corrosion resistant spring steel
(spring steel S) identical with example 1 was used. The manufacturing
processes are common to example 1 except for the shot peening conditions.
In comparative example 1, in a first shot peening process, the spring
wire was applied with the first shot having a shot size of 0.87 mm at a
velocity of 76 m/s. The processing temperature was 230° C.
Thereafter, in a second shot peening process, the spring wire was applied
with the second shot having a shot size of 0.67 mm at a velocity of 46
m/s. The processing temperature was 200° C.

[0062]FIG. 8 shows the compressive residual stress distribution of
comparative example 1. As shown in FIG. 8, the maximum value of the
compressive residual stress of comparative example 1 bears comparison
with examples 1 to 3. However, in comparative example 1, a part B' at
which the compressive residual stress equivalent to the compressive
residual stress value A' at the surface of the spring wire exists is
located at a position considerably shallower than the permissible pit
depth Rmax (0.25 mm). As a result of this, there was the possibility
of a fatigue crack occurring at a bottom part or the like of a corrosion
pit, and the coil spring being broken when the corrosion pit grew close
to the permissible pit depth (0.25 mm).

Comparative example 2

[0063]Comparative example 2 is identical with comparative example 1 except
that SAE 9254 was employed as the steel type for the spring wire. In
comparative example 2, the spring wire (hardness: 53.5 HRC) constituted
of SAE 9254 was subjected to a first shot peening process (velocity: 76
m/s, processing temperature: 230° C.) by using the first shot
having a shot size of 0.87 mm. Thereafter, the spring wire was subjected
to a second shot peening process (velocity: 46 m/s, processing
temperature: 200° C.) by using the second shot having a shot size
of 0.67 mm.

[Fatigue Test Result]

[0064]FIG. 9 shows results of corrosion fatigue tests of examples 1 and 2,
and comparative example 1. In the corrosion fatigue test, the coil spring
was subjected to test cycles each of which is constituted of subjecting
the coil spring to spraying of salt water (5% NaCl) for 30 minutes,
thereafter subjecting the coil spring to vibration 3000 times, and then
keeping the coil spring in an environment having humidity of 95% for 23
hours until the coil spring is broken, and the total number of vibration
was measured. The test stress was 1200 MPa.

[0065]As shown in FIG. 9, in example 1, the corrosion life was markedly
improved to 123% in comparison with the corrosion life (100%) of
comparative example 1. Furthermore, in example 2, the corrosion life was
largely improved further to 145%. As described above, in the coil springs
of the above examples 1 and 2, the corrosion durability of the suspension
coil springs was largely improved by the warm peening using the
large-grain shot having a shot size of 1.0 mm or more.

[0066]In the conventional suspension spring, when the corrosion pit grows
to approach the permissible pit depth Rmax, a crack occurs earlier
at a bottom part of the pit and the crack grows rapidly, thereby breaking
the spring. Conversely, in the examples described above, compressive
residual stress exceeding -400 MPa is imparted even to a deep position
exceeding the permissible pit depth Rmax. Furthermore, in the
examples described above, the compressive residual stress value at a
position exceeding the permissible pit depth Rmax is equivalent to
or greater than the compressive residual stress value at the spring
surface, and it is possible to prevent the gradient of a change in the
compressive residual stress from becoming steep. As a result of this, in
each of the coil springs of the examples according to the present
invention, even when the corrosion pit grows to reach the permissible pit
depth Rmax, the compressive residual stress still remains at the
depth, and hence it is possible to suppress the occurrence of a crack
from the bottom part of the pit. Accordingly, even when a crack has
occurred, it is possible to slow the subsequent growth of the crack,
whereby it is possible to tremendously improve the corrosion durability.

[0067]FIG. 10 shows results of corrosion fatigue tests of examples 4 and
5, and comparative example 2. Here, the corrosion life of a case where
the corrosion fatigue test of the coil spring of comparative example 2 is
carried out at the test stress of 1100 MPa defined as 100%, and corrosion
lives of cases where tests are carried out at the test stress of 1200 MPa
by increasing the test stress by 100 MPa are compared with the corrosion
life of above comparative example 2. When the stress was increased from
1100 to 1200 MPa, the corrosion life was lowered to 65% in comparative
example 2. Conversely, in example 4, the corrosion life of 104% which was
more than the conventional one was obtained. Furthermore, in example 5,
the corrosion life was largely improved to 160%.

[0068]As described above, in the coil springs of examples 4 and 5, even
when the employed stress was increased by 100 MPa, it was possible to
make the corrosion durability equivalent to or higher than the
conventional coil spring. As a result of this, it became possible to use
the suspension coil spring at higher stress, and effect weight reduction.
For example, in the conventional spring of the 1100 MPa class, the wire
diameter was 12.1 mm, total number of turns was 5.39, and weight was 2.09
kg, whereas in the spring to be used at 1200 MPa, the wire diameter is
11.7 mm, total number of turns is 4.93, and weight is 1.79 kg, whereby
weight reduction of 14.4% is achieved. In the spring to be used at 1300
MPa, the wire diameter is 11.4 mm, total number of turns is 4.61, and
weight is 1.58 kg, whereby weight reduction of 23.4% is achieved.

[0069]In the compressive residual stress distribution that can be acquired
by the conventional shot peening, even when it is attempted to produce
compressive residual stress greater than -400 MPa up to a depth of about
0.25 mm, attenuation gradient of the compressive residual stress from the
surface in the depth direction is steep, and hence the compressive
residual stress at the surface must be made extremely high. As a result
of this, it has been difficult to realize the above attempt due to the
limit in the manufacturing method. Further, it is not impossible to
produce compressive residual stress up to a deep region by using the
conventional shot size and by making the velocity higher. However, in
this case, in order to make the kinetic energy of the shot twice, it is
necessary to increase the velocity from 78 m/s (impeller rotational speed
2300 rpm) to 109 m/s (impeller rotational speed 3279 rpm). As a result of
this, problems of an increase in noise or vibration, increase in power
consumption, increase in wear of the equipment, and the like are caused.
Furthermore, in view of the manufacturing cost, increasing the velocity
of the shot is not suitable for mass production (practical application).
Further, in the conventional shot peening, the magnitude of the
compressive residual stress at the bottom part of the corrosion pit is
markedly and relatively lower than the magnitude of the compressive
residual stress at the surface. As a result of this, even when the
compressive residual stress is produced up to a position in the vicinity
of the bottom part of the corrosion pit, this has little effect on
preventing a crack from occurring in the vicinity of the bottom part of
the corrosion pit.

[0070]Conversely, in the compressive residual stress distribution of each
of the examples according to the present invention, large compressive
residual stress (greater than -400 MPa) is imparted to a deep potion
exceeding the bottom part of the corrosion pit. Furthermore, the
compressive residual stress near the surface of the spring, and
compressive residual stress near the bottom part of the corrosion pit are
maintained at the same level. Moreover, the gradient of a change in the
compressive residual stress is prevented from becoming steep in the
region from the spring surface to the bottom part of the corrosion pit.
By virtue of the above facts, even when the corrosion pit grows, it is
possible to effectively prevent a crack from occurring near the bottom
part of the pit or prevent the crack from growing.

[0071]Regarding the effect of each of the examples described above, the
same tendency is in evidence irrespectively of the steel types, in many
steel types including the above-mentioned high corrosion resistant spring
steel (spring steel S), SAE 9254, and in, for example, the spring steel
SUP7 conforming to Japanese Industrial Standards (JIS), the same result
was obtained. Furthermore, according to the present invention, it is
possible to enhance the corrosion durability by using an ordinarily used
spring steel for a suspension coil spring, and hence an effect of making
it possible to prevent the material cost of the coil spring from becoming
high is also obtained. It is possible to apply the coil spring according
to the present invention to suspension mechanisms of various vehicles
including automobiles.

[0072]Additional advantages and modifications will readily occur to those
skilled in the art. Therefore, the invention in its broader aspects is
not limited to the specific details and representative embodiments shown
and described herein. Accordingly, various modifications may be made
without departing from the spirit or scope of the general inventive
concept as defined by the appended claims and their equivalents.